Silicone rubbers are used in a wide variety of fields, for example, health care materials such as bottle teats and food-safe goods, hoses and gasket materials for automobile use, building members, and fiber coating materials because of heat resistance, freeze resistance, safety, appearance (transparency), touch, and durability.
Silicone rubbers used as gaskets such as O-rings and packings are required to have a low compression set and high heat resistance for preventing seal leakage. Typical silicone rubbers, however, have a substantial compression set and suffer from the problems of a change in shape and a significant increase in hardness during long-term service in high-temperature applications. In order to reduce the compression set and to suppress the hardness increase, often the rubbers shaped by heat curing must be further subject to secondary vulcanization at high temperature for a long time.
To solve the problems, Patent Document 1: JP-A H02-242854 proposes to add a triazole compound to an addition-curable silicone rubber composition for reducing the compression set thereof without a need for secondary vulcanization. Patent Document 2: JP-A 2006-225420 and Patent Document 3: JP-A 2004-190013 describe to add metal oxides and hindered amine compounds for suppressing any hardness increase, respectively. However, some of the cured products obtained by these methods have such physical properties as a compression set in excess of 30% as measured by a compression set test according to JIS K 6262:2013 using a specimen with a diameter of 13±0.5 mm and a thickness of 6.3±0.3 mm under 25% compression at 200° C. for 24 hours, and a hardness change in excess of +5 as measured by a heat aging test according to JIS K 6257:2010 at 225° C. for 72 hours. When these cured products are used as gaskets like O-rings and packings, the problem of seal leakage will arise.